The present invention relates to a pulverizer for obtaining fine powder, such as toner used for copying machines. More particularly, it pertains to a pulverizer comprising a housing and a rotor.
Pulverizers typically include a housing having an inner surface in which a large number of recesses are formed parallel to a center line running through the axis of the housing. The rotor is attached to a rotary shaft rotating at high speed around the center line, and has vanes or projections extending parallel to the center line so that there is a small space between the inner surface of the housing and the ends of the vanes. In such a structure, a powdered raw material, together with a large amount of air, is fed through an inlet at one end of the housing, and is pulverized and discharged through an outlet at the other end.
As shown in FIG. 8, a pulverizer conventionally includes a housing 16 and a rotor 18. The housing 16 is provided with an inner surface having a large number of recesses 17, each forming a triangle when viewed in section. Vanes or projections 19 are formed in the surface of the cylindrical rotor 18 so that there is a small space between the inner surface of the housing and the ends of the vanes.
A cyclone, a bug filter and an exhauster are connected in series to the back of the pulverizer. The exhauster blows air through the inside of the pulverizer.
Because of the many vanes or projections of the rotor rotating at high speed, an air stream of high velocity is generated around the outer periphery of the rotor and flows in the rotational direction of the rotor. The air stream is compressed when the ends of the vanes or projection approach the ridges between the recesses facing the rotor, and is expanded when the ends move away from the ridges. High-frequency vibrations are thus generated.
The powdered raw material fed into the pulverizer strikes against the vanes or projections and is accelerated. It then strikes against the surfaces of the recesses in the inner surface of the housing, which surfaces oppositely face a direction in which the rotor rotates, whereby it is pulverized into fine powder and then into finer powder by the high-frequency pressure vibrations generated around the outer periphery of the rotor. The pulverized powder, together with air, which has been discharged from the pulverizer is collected by the cyclone and the bug filter.
In recent years, there has been a demand for a toner for copying machines, 50% of which toner has a particle diameter of 10 microns or less which is finer than conventional toners. To obtain such fineness, the space between the rotor and the inner surface of the housing is made smaller, and the peripheral speed of the ends of the rotor is increased to 110-125 m/s.
The temperatures of the air and pulverized powder discharged from the inside of the pulverizer is increased significantly because of the tremendous disturbance of the air inside the pulverizer, the frictional loss of air caused when the high-frequency pressure vibrations occur, and because powder which has already been pulverized into the desired size strikes the vanes repeatedly.
It is necessary that the temperatures of the air and powder discharged from the pulverizer be maintained at about 50.degree. C. or less so that the powdered raw material cannot melt inside the pulverizer.
If the amount of air supplied to the unit weight of the powdered raw material is increased in order to limit the air and powder to the above temperature, the time is shortened during which the material is pulverized inside the pulverizer, thus resulting in incomplete pulverization.
The object of the present invention is to provide a pulverizer capable of remarkably increasing the amount of powder to be pulverized per unit time when the flow rate and temperature of air supplied to the pulverizer, and the temperatures of the air and pulverized powder are under predetermined conditions; and when powder having the same size as powder pulverized conventionally is produced at the peripheral speed of the rotor which is substantially equal to the peripheral speed of the rotor of the conventional pulverizer. The pulverizer of this invention is further capable of increasing the peripheral speed of the rotor to pulverize the powder into finer particles than powder obtained conventionally even when the powder is pulverized at the same ratio.